I'd say Karpov, and Seirawan gives a very convincing explanation in his book Chess Duels, I can post it here if you want.
Kasparov-Karpov 1984

I'd say Karpov, and Seirawan gives a very convincing explanation in his book Chess Duels, I can post it here if you want.
As long as it doesn't violate some sort of copyright, sure, that'd be great!

Karpov. My main reason is while Kasparov had been gaining on him, both were tired and Karpov was still winning. When play gets sloppy it'll usually favor the one with the better score at the start of it. I also think Kasparov would have run out of great opening prep to keep Karpov on his heels... But who really know for sure. They were both great players and the two without equals in the 80's.

Kasparov himself said that he was about to win, so they cancelled the championship. Karpov was in bad (physical) form and Kasparov was winning game after game on the end.

No, Kasparov said that he had chances to win, but that Karpov was still favored. Yes, Karpov collapsed horribly at the end, but he was still leading 5-3 with one win to go.

Do a bit of background research on who Florencio Campomanes was, and the issue will become clear. He essentially made a career out of helping the Soviets starting with the 1978 Karpov-Korchnoi match, in return for which he was made FIDE president. He terminated the 1984 match under pressure from the KGB and Soviet government when they saw that their boy was about to lose. And your source for this allegation is?
Not according to Edward Winter, a noted authority on chess biography and history. I used to agree with your viewpoint but don't anymore. BUT it certainly looked sleazy ending the match right after Karpov lost the last 2 games! For years afterwards I thought Kasparov would've won the next 3, but not anymore - and there are many GMs who seem to think Karpov would've won if the match wasn't terminated, inc Kasparov:
http://www.chesshistory.com/winter/extra/termination.html

Important political change happened in Kremlin during this match. Konstantin Chernenko died and Mikhail Gorbachev assumed power. Unlike Brezhnev, Gorbachev was fan of Kasparov rather than Karpov and he had let it become known. I guess Kasparov started receiving support overnight from grandmasters, therapists and fans, while the previous help to Karpov stalled or became half-hearted.
Karpov complained later that some of his close friends among grandmasters became open supporters of Kasparov instead.
[EDIT]
The match ended 8th February 1985, Chernenko died 10th March. But Chernenko had been gravely ill for some time and Gorbachev and his clique assumed power earlier.
Many (including Winter) point to Kasparov's own estimate of his chances to win at the time of the termination as smaller than Karpov's as proof that his chances were indeed smaller. But Kasparov couldn't know anything about how Karpov felt. If I recall correctly Baturinsky, who had been involved in Karpov's team for years, later said that he regretted that it had been impossible to make Karpov continue the match.
Karpov just didn't want to or wasn't able to continue the match, while Kasparov probably thought that starting again from 0-0 was better than continuing at 3-5, at least based on what he knew then. Still, it was Kasparov that complained, both then and later, about the termination, while Karpov had nothing negative to say about it in his autobiography or later, and he signed the agreement without protesting.
What Karpov wanted was a longer period of rest, to then continue the match from a 5-3 lead, just needing one more win. In such a situation he would have been a very probable winner. As it was my bet would have been on Kasparov, Karpov could easily have lost not just the last two, but the last three games played. Karpov didn't beat Kasparov during the last three months of the match, and was quite exhausted.

Kasparov himself isn't reliable source. He has been inconsistent in his acts, statements and memories. He joined Soviet communist party for opportunistic reasons, later he criticized (former) communists in Russia. He laughed at Campomanes when FIDE went bankrupt after split, but is own organizations vanished completely. His match vs Shirov never took place, he repeatedly broke his promises about unification matches vs FIDE champions etc.
I suspect he biased his memory here as well. No doubt - Karpov was exhausted, but so was Kasparov. He was pretty close to losing the sixth game several times despite he avoided risk. He didn't protest and was content with such favourable outcome, but he regretted it later influenced by his later performance.

"The KGB Plays Chess", by Boris Gulko, Korchnoi et al.
Winter may know biography and history, but when it comes to an intimate knowledge of the Soviet government and its machinations, I'll take Gulko & Korchnoi over Winter any day of the week.
There are two questions here: (a) Was the match terminated in order to save Karpov from defeat? and (b) what would have happened had it been allowed to continue? I think the answer to (a) is obvious. The only true answer to (b) is "we'll never know", but the pattern at that point sure wasn't looking great for Karpov, was it? That Karpov never uttered a peep in protest is very telling.
The obvious answer to (a) is "NO" despite Kasparov's later claims. Fact: Karpov filled an official written protest against such termination of the match. [Source: Československý Šach, March 1985] Hearsay: Kasparov allegedly suggested terminating the match himself during the long series of draws when he saw no chance to win. The termination of the match was discussed and indicated before the last few games took place and this may have caused Karpov's bad performance - he saw no reason to waste efforts. Much later Kasparov accused Campomanes & Baturinski they stripped him of great match triumph by behind-the-scenes machinations, because he overestimated these victories. Also Kasparov's fans seem influenced by these last two games a lot and suggest Karpov was in deep crisis and Kasparov was close to success.
You should also forget about any pro-Karpov clique in Kremlin as well. Karpov's fan Brezhnev had died more than 2 years ago and fresh wave of younger Kremlin's leaders prefered Kasparov.
The answer to (b) is Karpov would probably win. In his previous matches against Korchnoi he got tired and started losing towards the end, but he mobilized his will and the decisive final game he played always at full stretch. His final victory in the 32nd game in Baguio 1978 was impresive.
Karpov always supported whoever was in power at the moment. In the days of Brezhnev he claimed that Marxism was his main interest apart from chess. When Communism fell he immediately became a believer in capitalism and made millions in business, and is now a dedicated Putin supporter. If Putin falls Karpov will no doubt express his belief in the next leader as unquestioningly as in all the previous cases. I don't think he has been less of an opportunist than Kasparov, who always was quite critical of whoever was in power instead.
But that has less to do with if Kasparov's estimate at the time of the termination was right or not. Maybe he underestimated his chances at 25-30%, but I don't think he was too unhappy about the outcome though, considering that he had come back from 0-5 and avoided losing the match.
Karpov wanted to continue from 5-3 after a longer break, but he never complained about the termination in his autobiography or in any of all the interviews he gave. I don't agree with Polar_Bear that Karpov just didn't bother about those last games because he supposedly knew that the match soon would be terminated etc. The more reason for him to try to get his sixth win if that was the case. But he hadn't beaten Kasparov in three months by then, and this was no 20 years older Korchnoi he faced, like in 1978.
Some quotes and facts courtesy of Mark Weeks:
"On 1 February, Campomanes proposed to Yuri Mamedov, the leader of Kasparov's delegation, that the match continue for eight more games"
"Kasparov refused the proposal because the arithmetic was against him. Karpov would need only one win and could afford to take chances, while Kasparov would need four wins in the eight games"
"The Soviet chess federation, in a letter signed by V.Sevastianov, president of the federation, had demanded a three month suspension of the match, citing concerns about the health of the players"
"Campomanes and Gligoric visited Kasparov on the 14th to discuss Sevastianov's letter. Kasparov's response was that either Karpov should abandon the match or the match should continue. Mamedov was informed the same evening that the match was to be stopped"
"Campomanes stated that he had been with Karpov just before the conference and that Karpov wanted to continue the match"
"Kasparov was invited to speak and asked Campomanes why he was stopping the match if both players preferred to continue"
"Karpov signed, but Kasparov refused to sign"
http://www.mark-weeks.com/chess/84kk$$.htm

I got the impression soviet chess officials decided to terminate the match without any consent of players and against their will. They had serious reasons for it: with that many draws the public interest was decreasing, costs increasing, there were other scheduled events for staff persons and the place.
Soviet officials weren't used to ask for permission and players were expected to agree without hesitation. Florencio Campomanes was more or less their puppet, the only issues - make it look like general consensus of all parties: Soviet chess federation, FIDE, both players, their teams, designated match staff, and reach a reasonable compromise to resume WCC competition. And the problem appeared. With journalists from the West present, players revolted and refused to express consent openly - something inconceivable so far, having this happened in the Brezhnev era, they would have received ban to travel and play abroad. Karpov succumbed quickly to pressure and signed, but Kasparov resisted longer.
Maybe Kasparov really thought in retrospect the match was ended at Karpov's request to prevent Kasparov appear as winner. However the head of the Soviet chess federation Sevastianov was more Kasparov's fan than Karpov's. More likely it may have been exactly opposite: Sevastianov agreed to spare Kasparov from embarassment associated with decisive loss.

One of my points: It is not that clear who was the actual "Communist Golden Child" in 1984/85.
And yes, even communists can get fed up with something. They decided there was enough of chess as the match lost attraction with so many draws.

Kasparov was not above theatrical hypocrisy. According to everyone not in the Kasparov camp, Kasparov agreed to the cancellation of the match before his very public outburst at the press conference. My memory isn't what it once was, but I am pretty sure his signature is on the agreement to cancel.

I would like to post a link to a youtube documentary on karpov. It's in russian but it explains some of the things that happened in the karpov-kasparov matches. http://m.youtube.com/watch?v=lgZQCid7v-s

Kasparov anti-communist? You shouldn't smoke the weed that much. He was a member of the party the whole time up until Jelzin.
The decision to call it off had come before the last 2 games took place of course.
And btw, Karpov isn't pure ethnic Russian too. He has Siberian natives among his ancestors and maybe some Jews too.
Your book can't be accepted as reliable source. Korchnoi suffers with magical thinking and tends to see anti-semitic conspiracy everywhere.

Kasparov anti-communist? You shouldn't smoke the weed that much. He was a member of the party the whole time up until Jelzin.
The decision to call it off had come before the last 2 games took place of course.
And btw, Karpov isn't pure ethnic Russian too. He has Siberian natives among his ancestors and maybe some Jews too.
Your book can't be accepted as reliable source. Korchnoi suffers with magical thinking and tends to see anti-semitic conspiracy everywhere.
1. You're confusing having a Communist Party membership, which any Soviet citizen had to have in order to get anywhere, with being an ideological communist.
2. So why wasn't the decision reversed after those 2 decisive games?
3. Now you're just being dumb. Nobody is a 100% pure ethnic anything, but Karpov is as Russian a WC as you're ever going to find.
4. So is Gulko deluded as well? If you seriously doubt the institutional anti-semitism of the USSR, you're the one with the magical thinking.
1. Opportunistic communists have been even worse than ideological ones. Consistency matters. Karpov believed in marxism-leninism honestly as his poor father had. Kasparov joined the party just to be given better trainers and team seconds and to be permitted to travel and play abroad. Unlike Karpov, he was never true communist in his heart, but you can't call him an anti-communist. He was just among the worst sort of communist party members.
2. Who knows? Maybe because consistency matters. KGB was there of course and monitored all people around the match including both players. This was perhaps the most expensive and exhausting thing, so Sevastianov received strict instructions to call it off when the costs exceeded appropriated budget. KGB had huge resources but not infinite. You still ignore it was Karpov who wrote the official protest against it, not Kasparov.
3. Alekhine, Smyslov and Spassky were Russians at least to the same degree as Karpov - if we really do not count Russian Jews as true Russians.
4. We are getting on really thin ice here. To be honest, I don't consider literature about alleged anti-semitism written by Jews to be reliable and if I am not mistaken, Gulko regards himself as Russian Jew - his point of view can be biased while Korchnoi proved many times beyond any doubt his point was biased. Soviet Union wasn't openly racially anti-semitic except very short period just before Stalin's death. The problem of Jews in Eastern Bloc came when Israel turned against Soviets. Jews had to be monitored and avoided as potential spies and non-loyal citizens.
With the amount of hypothetical chess-history questions found on the forums on here, I'm surprised this one isn't asked more often : If the KK match of 1984 was played out to a finish, who do you think would win, and why?